This invention most generally pertains to the drying of waste sludge, and more particularly pertains to the drying of waste sludge where yard waste is mixed with the waste sludge before drying.
Municipal treatment of waste water creates large amounts of waste sludge. Waste sludge is what remains after the treatment of waste water by a number of known methods. Though water is recycled back to society, typically waste sludge is not and must be disposed in a safe, efficient manner. A few known examples of disposal methods are: by direct application onto land, by composting, by land filling, or by drying and incinerating the solids.
In November 1992, new laws were passed establishing governmental controls on sludge disposition. Sludges must now have at least 30% solids and have a complete pathogen kill before their disposal. Given these restrictions and the ever increasing cost of landfill space, it has become quite difficult for cities and businesses to discard sludge. What is needed are improved methods to recycle this waste.
A known alternative is burning sludge to produce heat. Waste sludge is readily combustible when it is dry. Normally in such procedures, the sludge is dried in a dryer and burned in a furnace where the off gas is passed through a heat exchanger. For an example, see U.S. Pat. No. 4,429,643. In such sludge treatment processes, wet sludge cake has a moisture content of about 80% as it enters the dryer. The moisture is flashed away and the dried product is pelletized. Any off gas from the dryer is mixed with outside air and used as combustion gas in the furnace to dry additional sludge.
In U.S. Pat No. 3,963,471 a process is described for producing uniform pellets in which wet sludge cake from a mechanical dewatering procedure is mixed with previously dried product pellets and the mixture is then cycled back through the dryer. It is described that the size of the pellets can be controlled by the amount of material recycled. The recycled pellets form nucleii for the formation of pellets in the dryer as they are surrounded by an adhering layer of wet sludge cake.
In U.S. Pat. No. 4,040,190 a process is described in which recycled materials are passed through a crusher before being mixed wit a wet sludge cake. In that patent it is indicated that the crushing device may be adjusted for controlling the size of the grains to be supplied to the device and that by crushing to a predetermined size the grain size of the output pellets can be controlled.
The present invention concerns a novel processing option to simultaneously recycle waste sludge and address an additional waste disposal problem. This additional problem is what to do with yard waste and arises because grass and leave clippings, in many cities, can no longer be publicly burned or sent to a landfill.